Fluorine atom-containing sulfonyl fluoride compounds having a fluoroformyl group (such as the following compound (i)) are useful as materials for ion-exchange resins. Heretofore, compounds having a fluoroformyl group have been synthesized by a process which comprises reacting perfluoroalkylene oxides to a cyclic compound obtainable by the reaction of tetrafluoroethylene with sulfur trioxide (SO3). for example, the following compound (i) can be obtained by reacting hexafluoropropylene oxide (HFPO) to the above-mentioned cyclic compound, as shown by the following formula. 
However, the conventional synthetic process was a disadvantageous process for practical industrial application, since a due care is required for handling SO3. Further, reduction of the price can hardly be accomplished because the difficulty in synthesis is high. In addition, the obtainable compound (i) is limited to a compound having a side chain (—CF3), whereby there has been a problem from the viewpoint of the membrane characteristics of the ion-exchange membrane prepared from a derivative of the compound (i).
The present invention has been made for the purpose of solving the problems of the prior art and provides a process which solves the difficulty in production and whereby fluorosulfonyl fluoride compounds having various molecular structures can be produced efficiently and at low costs.